In general, a vehicle implements various traveling controls. In many cases, such traveling controls of a vehicle are implemented based on a relation relative to a surrounding vehicle. For example, when a vehicle implements a rank-traveling control, the vehicle implements a speed control of the self vehicle according to speed control information about another vehicle in a rank, to which the self vehicle belongs.
When the self vehicle starts rank traveling relative to a vehicle (leading vehicle), which travels directly ahead of the self vehicle, it is necessary to specify the leading vehicle among vehicles, which travel ahead the self vehicle. A patent document 1 discloses a known art for specifying a leading vehicle. The patent document 1 discloses a configuration to detect the distance relative to a front vehicle according to GPS position information and to detect the distance relative to a front object according to a detection value of a radar detection device. The configuration of the patent document 1 further specifies a vehicle, at which the distance according to GPS position information coincides with the distance according to the detection value of the radar detection device, as a leading vehicle.    [Patent document 1] JP-A-2002-222491
In general, a number of vehicles simultaneously travel on a road. In consideration of this, it may be desirable to acquire, in addition to the behavior of a leading vehicle, the behavior of a vehicle ahead of the leading vehicle. More specifically, for example, it is assumable that a vehicle, which travels in front of several vehicles ahead of a leading vehicle, may decelerates. In such a case, when deceleration of the vehicle further ahead of the leading vehicle is recognizable, quick traveling control is enabled before the leading vehicle decelerates.
In a configuration equipped with an inter-vehicle communication device, behavior information may be receivable from a vehicle, which is in front of several vehicles ahead of the self vehicle and is within a communication range. Nevertheless, it is noted that, when a certain vehicle decelerates, deceleration timing and degree of deceleration vary among vehicles behind the certain vehicle. In consideration of this, in order to enable an advanced traveling control, it may be desirable, when behavior information is acquired from a certain vehicle, to determine the number of vehicles, which are traveling ahead of the self vehicle and traveling between the self vehicle and the certain vehicle.
Nevertheless, an inter-vehicle communication device is not necessarily equipped to every vehicle. Therefore, even when some of vehicles notify those current positions to surroundings via inter-vehicle communications, other vehicles, which are not equipped with an inter-vehicle communication device, may not notify those current positions to surroundings. Thus, in such a circumstance, a vehicle may receive notification from a part of surrounding vehicles, which are equipped with an inter-vehicle communication device, and cannot receive notification from other surrounding vehicles, which are not equipped with an inter-vehicle communication device. Consequently, the vehicle may not be able to recognize the other surrounding vehicles among all the surrounding vehicles, even receiving the notification from a part of surrounding vehicles. For example, it is assumed that vehicles 1, 2, 3 other than the self vehicle exist. In such a case, from the position of each vehicle acquired via inter-vehicle communications, it may be determined that the vehicle 1 travels directly behind the vehicle 2. Nevertheless, in such a case, the vehicle 3, which is not equipped with an inter-vehicle communication device, may exist between the vehicle 1 and the vehicle 2.
In addition, the configuration of the patent documents 1 may enable to recognize that a vehicle does not exist between the self vehicle and a leading vehicle, nevertheless, is incapable to recognize a state ahead of the leading vehicle.
In consideration of these subjects, even in a configuration to acquire a traveling state of a vehicle other than a leading vehicle via inter-vehicle communications and to implement traveling control of the self vehicle according to the acquired traveling state, a vehicle, which is not equipped with an inter-vehicle communication device, may still possibly exists. To implement the traveling control of the self vehicle, such existence of a vehicle, which is not equipped with an inter-vehicle communication device, need to be taken into account. Therefore, such a conventional configuration is hard to enable an advanced traveling control.